There are two downlink transmission modes for use in an orthogonal frequency division multiple access (OFDMA) system, i.e., a localized resource allocation mode and a distributed resource allocation mode.
The localized resource allocation mode allocates resources in the form of a gathered format on time and frequency axes in order to make easier channel estimation. The distributed resource allocation mode aims to increase time- and frequency-diversities, allocates resources in the form of a distributed format on time and frequency axes.
Using the above-mentioned localized and distributed resource allocation modes can increase the number of utilities of radio resources, so that the radio resources can be more effectively used by the above-mentioned localized and distributed resource allocation modes.
The localized resource allocation mode allocates resources in the form of a gathered format on time and frequency axes, thereby making easier channel estimation. In more detail, transmission (Tx) information of a predetermined number of terminals or UEs (User Equipments) is transmitted via a predetermined number of resource blocks (RBs).
In this case, the resource block (RB) is indicative of a collection unit of a predetermined number of OFDM symbols. In the case of the RBs of the frequency domain, a predetermined number of sub-carriers on the basis of a frequency-resource allocation unit of OFDM symbols configure a single resource block (RB). For example, according to the 3GPP LTE scheme, 12 sub-carriers configure a single RB. According to the 3GPP2 scheme, 16 sub-carriers configure a single RB.
The localized resource allocation mode transmits data of a predetermined UE using at least one RB, so that strong cross-correlation characteristics of the frequency selective fading exists between sub-carriers contained in a predetermined RB. The localized resource allocation mode performs frequency-domain scheduling based on an Adaptive Modulation and Coding (AMC) scheme using the aforementioned storing cross-correlation characteristics, such that it can be more effectively applied to low-speed or medium-speed UEs, each of which can acquire a multi-user diversity gain.
In order to increase frequency and time diversities, the distributed resource allocation mode allocates resources in the form of a distributed format on time and frequency axes, such that a predetermined amount of information corresponding to a single RB from among transmission (Tx) information of a predetermined UE is distributed over all frequency-domain resources.
From the viewpoint of either a high-speed UE having difficulty in expecting a multi-user diversity gain or a signaling overhead, the distributed resource allocation mode distributes transmission symbols, which are associated with undesirable-scheduling UEs and channels incapable of controlling the AMC control, to several frequency domains, such that it provides a frequency diversity gain related to the frequency selective fading.
The distributed resource allocation mode is classified into a frequency division multiplexing (FDM) scheme and a time division multiplexing (TDM) scheme according to distributed resource allocation categories of time-frequency resources for a frequency diversity gain.
Two methods for transmitting data of UEs for the distributed resource allocation have been used, i.e., a resource block level (RB-level) method and a sub-carrier level (SC-level) method. The RB-level method allocates transmission resources to individual RBs in different ways according to the localized resource allocation mode and the distributed resource allocation mode. The SC-level method performs puncturing data allocated by the localized resource allocation mode, and transmits data of UEs for the distributed resource allocation mode (hereinafter referred to as distributed-mode UEs).
If the localized-mode UEs and the distributed-mode UEs are mixed in an OFDMA downlink, a Node-B or base station (BS) must transmit suitable control information to several UEs, such that resources for data transmission are distributed to the UEs. A detailed control information structure and a control-information transmission method capable of supporting the multiplexing are not defined in a current OFDMA downlink. General or conventional control information cannot support data reception of the localized-mode UEs or the RB-level or SC-level multiplexed distributed-mode UEs.